1. Field of the Invention
The present invention relates to a technique for the Time Compression Multiplexing (TCM) of three TV signals and, more particularly, to a technique for simultaneously transmitting three fields of a television picture signal by a Time Compression Multiplexed signal where one field is transmitted as is, but time compressed, and the other two fields are transmitted as differential signals also time compressed such that all three fields fit into a single field period.
2. Description of the Prior Art
The tranmission of two or more high quality television (TV) signals through a particular band-limited channel as, for example, a satellite transponder of 36 MHz continues to be a challenge in optimizing the use of available channels or transponders in current as well as in the future systems.
One technique to better use such channel bandwidth was originally proposed in the article "Time-Frequency Multiplexing (TFM) of Two NTSC Color TV Signals--Simulation Results" by B. G. Haskell in BSTJ, Vol. 60, No. 5, May-June 1981, at pages 643-660, for transmitting two broadcast quality color television signals on terrestrial microwave radio facilities. With TFM as then defined, two successive lines, or fields, of one picture would be frequency multiplexed so that they could be sent in one line, or field, period. During the next time interval, two successive lines, or fields, from the other picture would be transmitted, thus avoiding any crosstalk between pictures. In order to reduce the bandwidth required for the TFM signal, one of the two simultaneously transmitted lines, or fields, would be sent as a bandlimited differential signal, modulated onto a vestigial sideband (VSB) carrier frequency situated above the baseband video. The resulting TFM signal could then be transmitted through the microwave radio facility via narrow band FM.
In the article "TV Bandwidth Compression Techniques Using Time Companded Differentials and Their Applications To Satellite Transmissions" by K. Y. Eng et al in BSTJ, Vol. 61, No. 10, December 1982, at pages 2917-2927, it was proposed to combine the concepts of time compression multiplexing (TCM) and differential signals to transmit up to three National Television System Committee (NTSC) color TV signals over a satellite transponder. More particularly, in accordance with the article, it appears possible to transmit three TV signals per transponder using a line differential compression method and a 30 meter earth station antenna or a field or line plus field differential compression method and a 10 meter antenna.
The problem remaining in the prior art is to provide better use of the frequency spectrum by permitting the simultaneous transmission of three TV signals during a single field period of a television picture signal.